This invention relates to a method for treating an aqueous dispersion of microcapsules, and more particularly to a method for removing residual formaldehyde from an aqueous dispersion of microcapsules each having a capsule wall of an aldehyde polycondensation resin formed by polycondensation of formaldehyde and an amine in the presence of an acid catalyst.
Microcapsules find their usefulness in various fields such as pressure-sensitive copying papers, heat-sensitive recording papers, adhesives, fertilizers, pharmaceuticals, foods, cosmetics, etc. There are various known methods for making microcapsules useful for those purposes. Among them, there are included the "coacervation" method, the "interfacial polycondensation" method and the "in-situ polymerization" method. It is also known that microcapsules each having a capsule wall of an aldehyde polycondensation resin formed by polycondensation of formaldehyde with an amine are superior in the water resistance and the solvent resistance. For example, U.S. Pat. No. 3,516,941 and South African Patent Specification No. 62-939 disclose a method of making microcapsules in which urea-formaldehyde polycondensation resin capsule walls are formed through the utilization of the "in-situ polymerization" technique. Japanese Laid-Open Patent Publication No. 9,079 of 1976 discloses another method of making microcapsules in which urea-formaldehyde polycondensation resin capsule walls are formed by utilizing a system modifier. Japanese Patent Publication No. 12,380 of 1962, Japanese Patent Publication No. 12,518 of 1963 and Japanese Patent Publication No. 4,717 of 1973 disclose some methods of effectively performing aldehyde polycondensation reaction at an oil-water interface by utilizing chemical or physicochemical bonding. UK Patent Specification No. 1,190,721 discloses another method of making microcapsules in which complex capsule walls are formed from a suspensible polymer and a hydrophilic polymer by a reaction of a polyhydroxy phenol with an aldehyde. Japanese Patent Publication No. 51,714 of 1972 discloses a method for forming polycondensation resin capsule walls by precipitating polyvinyl alcohol and a polyhydroxy phenol through the utilization of a face separation agent and then carrying out polycondensation with an aldehyde. Japanese Laid-Open Patent Publication No. 57,892 of 1973 discloses a method of making microcapsules in which calsule walls are formed of a polymerization product obtained by coagulation reaction between an aldehyde and a polyhydroxy phenol complexed with polyvinyl alcohol. Japanese Laid-Open Patent Publication No. 20,069 of 1972 discloses a method for polycondensation of polyamine and an aldehyde through the utilization of the "interfacial polycondensation" method.
Among aldehydes useful for forming aldehyde polycondensation resin capsule walls in the above-mentioned methods, there are included formaldehyde, acetaldehyde, butyraldehyde, crotonaldehyde, benzaldehyde, glutaraldehyde and furfural. Formaldehyde is most preferably used because of its good wall and film forming ability. However, on the other hand, the utilization of formaldehyde is disadvantageous because of its pungent odor and poisonous character. The laws provide strict restrictions for the utilization of formaldehyde. It is therefore desirable to prepare an aqueous dispersion of microcapsules in such a manner that substantially no residual formaldehyde exists therein. However, in order to form capsule walls having the desired good properties it is essential to utilize an excess amount of formaldehyde, consequently the existence of a certain amount of residual formaldehyde in the microcapsule dispersion is inevitable. Apparently, this greatly restricts the usefulness of the microcapsule dispersions.
An attempt has been made to remove the residual formaldehyde from an aqueous dispersion of microcapsules having capsule walls of an aldehyde polycondensation resin. Japanese Laid-Open Patent Publication No. 5,874 of 1979 discloses a method of removing an excess amount of formaldehyde from an urea-formaldehyde polycondensation capsule wall microcapsule dispersion by adding thereto hydroxylamine sulfate and then adjusting the system in an alkaline state. In this method, the agent for removing formaldehyde is limited to hydroxylamine sulfate. In addition, according to our study with some experiments, it has been found that the adjustment of the microcapsule dispersion in an alkaline state tends to reduce the solvent resistance of the microcapsules.
The principal object of the invention is to provide an improved method for removing residual formaldehyde from an aqueous dispersion of microcapsules each having a capsule wall of an aldehyde polycondensation resin formed by polycondensation of formaldehyde with an amine in the presence of an acid catalyst.
Another object of the invention is to provide a method for effectively removing residual formaldehyde from an aqueous dispersion of microcapsules of the kind described by a chemical modification treatment without sacrificing the superior properties of the capsule walls.
A further object of the invention is to provide a method for accelerating the reaction to remove residual formaldehyde from an aqueous dispersion of microcapsules of the kind described.
Other objects and advantages of the invention will become apparent from the following detailed description.